Rob Ostapiuk version 4.11.2 of Syntax Highlighting should fix the problem. I have a clean XWiki 18.8.0 and syntax highlighting works fine both on the main wiki and on subwikis. Maybe you have a caching issue. Have you tried a forced-refresh or cleaning the browser cache?
Apparently a colon ':' isn't getting changed to a '/' and instead is getting encoded as '%3A' which breaks the URL.
No, %3A is perfectly fine here. 'wiki:tools' is the namespace, and it gets URL encoded in the URL path. BTW, if you upgraded to v4.11.2 then the CodeMirror URLs should look like this:
